what is mechanism that produce color in butterfly wings? pigment or diffraction ?
Butterflies get their colors from two sources: pigmented color and structural color.
E.g. Butterflies get their pigmented brown and yellow colors from melanin.
Structural, iridescent, colors comes from light interference on the scales of butterfly's wings.
Some colors are result of both effects: for example yellow pigment underneath a structure that creates a blue iridescent color, may create a green shade, made by the merging of the two colors
Dear Abbas,
About 20 years ago, it turned out that nature has many millions of years can create color and without special colored substances - only at the expense of ordered structures is very small (nanosizes) . This mechanism of staining, in contrast to the "chemical", based only on optical principles. When light is reflected from nanoelements structured in multilayers - lattice, lace, grooves, since the size of these elements are comparable with the wavelength of light, there is interference, diffraction and scattering of waves - as a result we see color. This color optical origin called "structural ". It turns out that it, along with the usual naturally occurring quite often - in insects, birds, fish, shellfish and marine plants.
In the animal world there are three kinds of color: only structural (butterfly Morplo), only pigment (like butterflies Gonepteryx) and structural combined with pigment. The blue color of the wings is often created structural coloration due to the scales, but if it is added to the yellow pigment, then there is an additional green.
More information you can find in:
P. Jucusic, J. R. Sembles. Photonics structures in biology. Nature, 2003, v. 424, p. 852.
Best regards, Leonid
Dear Sergiusz , Leonid and Hrvoje ,
Why gold is yellow in color?Due to absorption or emission wavelength of yellow ?
Very interesting question. Indeed, it would seem that all metals have to have a silvery color. However ...
In short, it can be explained as follows. indeed, the most remarkable feature of gold is its intense yellow colour as compared with the grey-white of silver. Why is that so? On metals the orbitals broaden to bands which extend over the whole crystal. The most exterior electrons are on the so-called valence band. On gold these are the d-electrons whose energy by indirect relativistic effects - as described - will be increased. Thereby the whole valence band will be raised. Above it - as usually on metals - is located the conduction band. This is formed on gold by s-orbitals. Here the energy will be decreased by direct relativistic effects and the band will be lowered. Thus the distance between these two bands will be decreased. On silver it is still so big that only ultraviolet photons could lift an electron from the valence band to the conduction band. On gold these two bands are so narrow that even photons with less energy from the blue range of the spectrum can do this. Hence gold absorbs blue light and appears to our eyes in the complementary colour, the intense yellow.
I note only that the modern view yellow gold is associated with the manifestation of relativistic effects. If you were to do a solid state calculation on gold without including relativistic effects you would predict it to be silvery. Including relativistic effects you get reasonably good agreement with reality. Relativistic theory of atoms and molecules gives a nice overview of this and many other phenomena. More information can be found at:
1. https://www.fourmilab.ch/documents/golden_glow/
2. http://math.ucr.edu/home/baez/physics/Relativity/SR/gold_color.html
3. "Relativistic Band Calculation and the Optical Properties of Gold", Christensen N E and Seraphin B O 1971 Phys. Rev. B 4 3321
4. http://www.webexhibits.org/causesofcolor/9.html
5. http://www.sciencedirect.com/science/article/pii/S0301010404005270
6. http://onlinelibrary.wiley.com/doi/10.1002/1521-3773(20021004)41:19%3C3573::AID-ANIE3573%3E3.0.CO;2-R/abstract;jsessionid=EBD2B0C0D36CBE375F3C4D66C7317CFB.f04t03?deniedAccessCustomisedMessage=&userIsAuthenticated=false
7. http://pubs.acs.org/doi/abs/10.1021/ar50140a002
8. http://scitation.aip.org/content/aip/journal/jcp/122/16/10.1063/1.1884985
There are a lot of interesting articles on the relativistic theory of atoms and molecules
Regards, Leonid
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